With three biotech companies gearing up to begin clinical trials of their embryonic stem cell therapies, the FDA convened a panel yesterday to debate the safety of these therapies, the biggest concern being that these novel cell therapies carry a risk of cancer.

According to a stem cell blog called The Niche:

Three companies, Geron, Advanced Cell Technology, and Novocell, described their work bringing embryonic-derived cells in (respectively) acute spinal cord injury, visual impairment, and diabetes. One expert who wasn’t on the committee said that the discussions had been impressively grounded in science, even getting into specifics about what assays might be considered. Attendees were surprised that no opponents of embryonic stem cell research showed up, but the FDA’s announcement said explicitly that it was only the cells’ safety that was under consideration.

The dark shadow of gene therapy looms over the regulators–the field suffered a major setback in 1999 when a patient died of cancer linked to the therapy. Scientists know that undifferentiated stem cells can form into a benign mass known as a teratoma when injected into animals, and they fear that a safety incident in the first round of clinical trials could devastate the already-troubled field. The cell therapies under development use differentiated cells, but the possibility remains that some undifferentiated cells may be left in the mix.

According to The Niche, major questions need to be answered to assess that risk:

How do we know what cells we have? How do we know what the cells will do in the body? Where do you put cells? Where do they go? What do they do? How many cells might be dangerous? How many can be useful? What can animals tell us? If the cells “go rogue” in a human participant, will we be able to stop them or even to track them? What’s the best way to balance risk and benefit?

The committee declined to speculate when it would release its guidance statement. But Geron has said it plans to begin trials of its cell therapy for spinal cord injury this summer.